Receiver fidelity control system



e the intermediate frequency (I.

PatentedJulyA 1945 ,UNITED sTATEsPA EECEIVERFIDELITY CONTROL SYSTEM f Winfield n. Koch, HauaonseldN. J., assignor l Radio Corporationlof America, `a. corporationl foy of Delaware l Application July 4, 1942, serial No. `449,771'` l. l i (Crest-6) qv i Y 4 o e isolalms. My present inventionfrelates to `a fidelity con# l trolsystemforyradio receivers, and more par-.- -ticularlywto a novel method of, and means for, .o

controlling the audio pass `band width of ai re-` ceiver of angular velocity-modulated carrier e Waves.

(FM) transmissionthere is employed alchannel Width of `200 kiloicycles '(kc'J. `The FM band` itself extends from 42 to 50 megacycles (me). l The carrier at the transmitter is actually `deviated over a range up to a maxilnumlof` 75 kc. on either side, of the carrier, or center, frequency` In other Words,thechannel Width is considerably in excess of the audible modulation frequency range. l

1 'It may be statedfthat it is one of the -main objects" of'niypresent invention to provide a radio transmission system wherein the receiving system `is controlled so as to accommodate the pass band Width of theaudio channel section of the receiverto the frequency range of the modulation on the carrier vso as to prole/idea high degreeof delityof'reproduction at the receiver.` V

"Alll her jimportant; objectof the invention is to p'r ide anFM receiver which includes a circuit o for automatically accommodatingithe audio` amplifier pass `band tothe vrate 'of frequency `deviation of the received carrierenergyV thereby to provide a audio tono `Control for jthereoeiter. i

Yet another object of the ini/"ention` isito pro- Vide asi/'stem of radio communication wherein l carrier wave' energy is frequency modulated with desired program modulation signals, the modulation ofthe carrierincludingin additionalJ super- -audible modulation signal Whosefunction it is, to

present frequency `modulated carrier Wave oeivers whichv are economical to manufacture and iiassemble. l Y. l q l The novel features WhichoIbelieve tobe char- `acteristic. of'lmy invention `are set forth with partcula'rityA in the appended claims; the. inven.- l tion itself, however, as `to both its organization l and .method ofioperationl Will best be understood by reference to vthe following .l description taken Vin connection with the drawingv in whichI have indicated vdiagramm'atically l several i. circuit organizations whereby my invention may be` carried Q l .intoeffectpw l f l lf UInthe drawingzf Fig. l `shovvs schematically an transmitter embodying the inventon`,`

Fig.` 2"shoWs themo ulation characteristic of l l the pilot oscillator attheltransmitter, o I Fig; 3 schematically shows thereceiverlusedin the present system.

characteristic of the receiver. l 4 Referring, now', to the accompanying `draW- o Fig. dshjows 'the control .tone discrimination ing, Fig. 1`schematicallyrshowslthe various net# 80., i Which isi` coupled to the audio xfrequencyampli- Works of an FM transmitter`.j\'1his invention is in no Way restricted to FM,L. since litcan be readily applied to phase modulated carrier Wave energy. Y

The expression fangular Velocity-modulated carrier waves is to be understood asgenerically denoting either frequency or phase modulated car` rier Waves; e The source of" `audio "lwave energy fierl maybe "a microphone; phonograph'record pick-up,` line or any yother` .typelof audio modu-r lation source. Further," the `modulation ,Signals need not be at audioffrequency, since modulation frequencies-off'aisuperaudible frequency range can be used. i `The control tonein that "case may be at some freqencyfabove the operating modula-l tion signal'frequency-`range. i i is amplified `at l r The modulation `Signat `energy ,oy and the amplified modulation energy isusedfftofvary'the reactive eifect of a frequency modulator 2.1 The latter, as is Well known in the FM transmission art`,`may consist of atube Whose anodel to cathode impedance simulates a `l reactarice 'whose signdependson the circuit con-i` `controlftlfe audio pass blandwidthxat the receiver so' as' toprovide an audio channel Width which corresponds directlyjto the frequencyrange of the inodulationsignals." l

`Stilliother objectsare to improve generally the ellioenoyf and noleutyof transmissiongand lator `3. oscillations, in the absenoe of. mooulatlonfnavea carrier frequency nections".` `l'Ihe l applicd'"modulation,` usua1ly"` applied toa control gridof thereactancetube, varies the sirnulatedjjreactanceg` {The flatter ,isjcon-f nected acrossgthe tank `circuit of themaster oscillThe tank circuit is. tuned so thatthe iooatodin tnogflz toto mo. band. `"The latter` lis the presen 1y assignedglFM broad-i` cantano.: y

' pass 'lter 9` and a diode rectifier II.

agev limiter IIlis interposed between the high` y Of course, the inventionis in no way restricted tov operation in the 42 to 50 mc. band. Any higher, or lower, frequency band may be utilized. What is important is that the carrier frequency, say 49.9 mc. for example, be deviated in proportion to the amplitude of the modulation signals. The rate of carrier, or center, frequency devial tion is afunction of the modulation frequencies per se. Inthe present FM band of 42 to 50 mc. it is the practice to deviate the'carrier frequency a maximum of 75 kc. to each side of the center of 200 kc. The FM wave energyis transmitted to a power amplier 4, and then lis-radiatedfrom the radiator 5. The lattermayybea dipole, or a.

grounded antennal system.l

- In my application Serial No. 444,639,1iled May 27, 1942, I have shown a methodiofin'cludng vin the modulated carrier .wave energy a super'- audible frequency control tone.

purpose of including-the control tone was to em- In that case the arranged in the space current path of diode I2.

, Each of load resistors I3 and I4 is bypassed by frequency. The FM channel itself has a width a condenser having a low impedance-to ,audio` frequency currents. 'Ihe anode end of load resistor I4 is grounded, while the cathode end of 4 the resistor is connected to the anode end of load l resistor I3. The cathodeend of resistor I3 is connected to the controlgridfof the reactance tube of the modulator l. IThe resistor-condenser filter network I5 is inserted in the control connection path between the loadv resistor I3 and the modulator l.

The voltage developed 'across each of yresistors I3-and I 4 will depend upon the audio energy passing each'of the input filters. Thus, when` audio energy inthe program modulation has no Afrequencies present greater than 3,000, cycles,

ploy a receiver having -selector circuits'which iliary 'frequency modulator 1," which maybe a reactance tubeof the sametype as is vemployed at the modulator 2, is utilized to vary the frequency ofthe pilotoscillator 'tank circuit. The reactiveeffectidue to the modulator 1 is controlled in accordance with the audio frequency modulation applied to the modulator 2 Thus, theaudio frequency amplifier I feeds a portion of its modulation `energy output to. a special amplifier 8. The output of "amplifier 8 is passed through a pair ofnparallel filterl networks 9 andl.

The network 9 is a high pass lter having a cut-off' frequency at Bfkc. The network I0 is a high pass filter having a cut-off frequency at 5 kc. In' other'words`,lthe'energy at the output terminals of the .high passv lterS- willrextend from '3,000c'ycles-and` upwards. VTheiilter I0 will pass audioenergy only above 5,000 cycles. AA voltage limiter 9 is interposedbetween the high The voltpassfilter I0 and its associated rectifier I2. The purpose of the voltage limiters isr to suppress any amplitude variation which may exist in theI audio energy thereby feeding to each of the rectiers II and I2 audio energy whiclrhasv a predetermined voltage when certainl'high frequency components' are present. Those skilled. in the art are fully aware of the manner of constructing the l high passlters 9 andflll. Itis -well known in the art of audio frequency transmissionthat audic'filters can be designed to have a sharp cut-offfrequency at any specified audio frequency value.' Of course, each of the high pass lters 9 and 'Ill may'bereplaced by a `lcandpass filter designed to pass Vthe desired audio frequency bandi.`v- M The rectifier Il has its anode coupled to the audio'tr'ansformer ll',\while`rectiier I2 has its anode 'coupled to the audio transformer I 2, The load resistor I3"is arranged'in the space;l current path of diode II, while the load resistor I4 is there will be no audio energy passed to either of vrectifiers VII or l2` Hence, the frequency deviation of the carrier will be determined solely by the Venergy iiowingfrom amplifier I'to vmodulator 2. When the programmodulation-has-no frequencies present greater 'than' 5,000-` cycles, there will be passed throughflter Ail audioenergy", since the lter passes audio frequencies 'above 3,000 cycles. There will be no transmission'of audio energy through `filter I0; because the-latter transmits audio energy above 5,000-cycles.'v As aY result voltage will -be developed across resistor I3 l which is positive with respect to ground'. This voltage is applied to the control grid `ofthe FM reactance tube Ifin a sense to increase' the frequencyof the pilot oscillator I5v up' to 19' kc:

. If, now, the`l` program modulationhaschigh frequencies above 5,000cycles,v audio energy will pass through filter I0', and, after limitinga't I0",

will be rectified by rectifier I2: As a resultthere will be developed. additional positive voltage across resistor- I4, and thev frequency modulator-1 will'cause" the oscillator 6 to shift in frequency up to 20 kc. The filter I5y functions to prevent pulsatingvoltage components Yfrom, passing to the modulator tube 'I. Itv will be seen, reference being made to' Fig; 2, that the pilot oscillator-fre.- quency'isa function of the audio pass .bandifrom the audio source. The 'wider theaudiofrequency range of the audio energy, .,thefhighenwill be the frequency of'the pilot oscillator. .The output of the pilot oscillator is fed over lead I6 to the frequency modulator 2; so that the` current variation oi the latterv will includerthe pilot frequency asa component. It'will now be appreciatedvthatfas the` applied audio frequency energy froml the audio source causesvthe carrier to deviate,l there is concurrently embodiedI in the FM energy a component for determining the mode andextent of audio pass band width to .be employed 'attherey ceiver. The control componentI.corresponds.to.a specific rate of frequency, deviation lof .the carrier which is proportional. to. `the audio frequency range of the .program modulation, At the receiverthere may beemployed the-usual FM receiving circuit.V The collector I1.f,eedsthe collected FMwave yenergy tothe usual/networks up to therdemodulator designatedasvFM Detl The demodulator maybe preceded bythe customary` amplifiers. Where the receiver. is k0f the superheterodynef type, as `is universally. the case,

the amplifier I8..may embody thefusuallnumber of ultra-.high frequency amplifiersv feedingacon; verter I9 whoseintermediatefrequency,(12F.) is passedthroughone or morefLF. amplierssZ followed. by amplitude modulation.. Emmer al.. The-limited I. F. .energy is fed to the FMdetector 22 to provide thev modulation signals which were u originallyapplied to the amplifier at thetransl fmitter. Each of the tunedselector' circuitslup to the detector 22 will have a pass .band` sufficiently wide to pass the maximum' frequency` deviation of thecarrier.. In other words, the tunedselector circuits should be in `excess of 150 kc. in width. Of4 course, the control system shown in my aforementionedapplication mayv be employedgand, in thatcase, a separate `super-audible frequency control modulation would be required on the carrier. The center frequency of the I.: F. energy may be,

for example,4.3 mc. The detected'FM wavefenergy ispassed througha stage of audio frequency amplification 23 prior to impression of the energy upon either of the plurality of parallel` audio frevquency amplifier `channels 24, 25 and-26. l .1

, The audio channel 241s .an amplifier capable l of passing energy of 301to=3,000 cycles efliciently.

Channel. 25 passes audio energylyingin the range of. .3,000 ,to;f5,000 cycles most efficiently. `Audio amplifier 26 passes audio energy from 4between 1 ,0.00 to 10,000 cyclesmost efliciently. The amplied audio output `of amplifier 123 is fed to the in- ,put terminals of each of channels24, 25 and' 2E. lf'Ihe output terminals of these three channels are `Aconnectedin common to feedthe input terminals of the audio power amplifier .21,V and thelatter will feed any desired `type ofreproducer. It is believed that thoseskilled in the art are` fully acquainted with the manner of constructing each of channels 24, 25 `and 2B. l `For example, `they maybe of one or morek stages in each channel.

. The audio amplier channel 24 has normal and Y. l invariable gain. In other words, `it is not subjected to gain control actionby the super-audible 1 control tone. i However, each of channels .25 and 2li is subjected `to gain control. This is accomplished by deriving from the output of the de` modulator 22 the modulation component correspondingto thelsuper-audible control tone. A band pass filter, capable of efliciently` passing'lS tokc., is designated by numeral 30.` It passes theicontrol tone energy to anamplifier 3|.` `It is now only necessary to convert the variablefrequency control toneenergy into a uni-directional voltage whose magnitude is a measure of the eectivefaudio channel width to be employed.

This is accomplished by employing a rectier 32 of )the diode type.

which is bypassed by the condenser34 for alterhating current components. The anode of diode 32 is connected back-to ground through the secondary winding 35 of transformer 36. The primarywinding of the transformer 36 is connected quency variation range of the control tone energy.

The cathode `of `diodet32 `is connected to ground through `a load resistor 33 mum at the 20 kc. frequency of the controlwtone :energlnand is a minimum at the 18 kc; frequency.

Amplifier lhas again controlconnection.40.1to

the cathode` end `of `resistor.;,33through a path which includes thenegative biasing `sourcell and the filter` resistor 42. The junction `of resistor l v' .42 and the positivezterminal ofsource4l iscon- `nected to groundthrougha filtercondenser 50.

Hence, amplifier 25 normally hasfa negative bias applied thereto which is determined bythe magnitude of biasingsource 4|.` Amplifier-Q23 hasa gain control connection 60 tothe cathodeend of resistor 33, `but the'biasing source forthajt jamtions `to the various networks. i

f In operation, `each of `channels 25and 26. will plierincludes biasing source 4| lin series with `biasing source 6|.` A s shown, sourcew4|1andj6| maybe asingle source with appropriateconnecnormally bench-conductive by virtue oftthe respective bias voltages applied `thereto over connections 40 and 60. Let it be assumedthatthe I "program modulation derivedfromlthe F'Mgdetector 22 has no frequencies above `3,000 ucyclesV present. In thatcase, the `super-audible control tone will have a frequency of 18 kc.n `Accordingly,- because of the discriminator .characteristic ,very little voltage is developed across 1 rectifier load resistor 33. Hence, the amplilerchannels `25 and 26` will be cut off due to the cut-off bias i appliedfrom sources 4| and (il.` f However,` where the program modulation l includes-frequencies above 3,000 cycles,.the discriminator character- 3 l istic shown in Fig. 4 will cause the rectifiereto develop increased positive voltage` across. resistor For audio.frequenciesbetween 3,000 and ,5,000 1 cycles and the corresponding control tonen of-19 kc. there willbe sufficient positive voltagetde- `veloped atthe cathodeuend of resistor33 `toovercome vtheb'ias due to source 4|, and, therefore,

y render channel 25uoperative. `As thenaudiol frequencies present increase "above 5,000 cycles` up to 10,000 cycles, the control tone willbecome20` kc., and there will be an increased4 inputtlevel at rectifier 32. `The positive voltagelacross` resistor 33 will then be sufcient to overcome the cut-off bias applied due to source "6L fHence,

channel 26 will be rendered operative.` Accordn f `ingly, it will be seen that in such `caselall: three channels will be operative, and there will be fedto `poweramplifier 21 a wide range offaudiofrequencies up to 10,000cycles. l l t l .Itwill now be appreciated thatthe audio pass band at the receiver is directly dependent 'upon the magnitude of the super-audible controltone frequency. Furthermore, it will be appreciated that vthe super-audible control tone frequencyyis a direct function of the audio frequency range In other words, discrimination takes place because the mean frequency of' the 'control tone energy is located at an intermediate portion of the sloping side of the response curve. "It will `be recognized that this is a well known and simple form of discriminator action. The output of dis- V Hcriminator 35-31 is a when the edn-.-` trol tone has a frequencyof `18 kc., and is a max- 1 imum when thefrequency is1of`20 kc.'

irnodulation at the transmitter. jmitter audio frequency range is expanded, there n will occur anautomatic expansion of the audio of the energy applied to the frequency modulator 2 at thetransmitter. In other words, the audio network of the receiverY is automatically accommdated `to the' frequency range ofthe audio As the transpass band width at the receiver.V Should. `the.

audlio frequency rangev becompressed at j the` transmitter, there will be an automaticcomprejssion of the `audioband pass at the receiver. As a result, highfrequency noises, whichwould otherwise occur and be reproduced, arepreven-ted" from reaching the speaker. On the other hand; whent program materiallof high fidelity is placed on the 'carrier at the transmitter; thereceiver `will autol .Hencethe voltageacrossresistor 33 amaxi- 2,379,720 system, producing frequency modulated carrier wave energy, embodying in the energy al control component Whose frequency magnitude is a controlled function of the frequency range of` the modulationv frequencies demodulating thefre,-

quency modulated carrier'wave energy at a `re ceiving `point to provide the modulation signalV l frequencies and said control component, and uti-` lizing said control component at thereceiving point to regulate the modulation range subsequent to demodulation.

12. In a `methodof signalling, deviating the frequency of carrier oscillations in accordance with modulation signals, generating a super-audible control tone, varying the `frequency ofthe control tone in accordance with changes in the frequency range of the said modulation signal frey quencies,modulating said carrier oscillations with said control tone, and transmitting the modulated carrier, oscillations. l

13. A radio communication method comprising `deviating the frequency of the carrier oscillations in` accordance with modulation signals, generating afcontrol tone whose frequency lies outside l the range of the modulationsignal frequencies,

varying the frequency of the control `tone intaccordance with changes in the frequency range of the said modulation signal frequencies, modulating said carrier" oscillations with said control tone, transmitting the modulated carrier oscillations, detecting the transmitted `carrier oscilla tions at a `receiving point, separating energy of `said control tone from the detected oscillations,

deriving from the control tone energy a control l e voltage whose magnitudeis dependent upon the frequency of said control tone,` and automatically adjustinglthe modulation pass band width atthe of the control voltage. l

14. In a receiver of frequency modulated carrier sponsive to variations in frequency of said control tone for renderingpredetermined ones of said channels effective.

f 15. In combination, means for generating carrier oscillations `of" high frequency; means for deviating` the frequency ofthe oscillations inaccordance with `modulation signals, means for generating a control tone Whose frequency lies outside the range of the modulationsignal frequen-` cies, meansfor varying the frequency of thecontrol tone in accordance with changes in thefrequency range of thesaid modulation signal fre` quencies, means for frequency modulating said carriergos'cillations with said control tone, means for utilizing the resultantfrequency comprising means for detecting the resultant modulated carrier oscillations, means for separating energy of said control tone from the detected oscillations, means deriving from the control tonegenergy a control voltagewhose magnitude is dependent l upon the frequency of said control tone, and means for automatically adjusting the modulation pass band Width subsequent to said detecting means in accordance with the magnitude of the control voltage. l l y l WINFIELD R; KOCH. l

receiving point in accordance with the magnitude 

